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Bluefin Tuna

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Bluefin Tuna Bluefin Tuna _Canada Underwater World 2 raditionally, the bluefin tuna It was 304 cm in length and 679 kg in T(Thunnus thynnus, L.) has been weight (Fig. 1). Bluefin Tuna considered one of the world's top sport The bluefin is generally dark, metal- fish. With its great size and power, in lic blue (nearly black) on the back, shad- addition to a streamlined body capable ing to silvery white or grey on the sides, of great bursts of speed, this spectacu- to white on the belly. Its somewhat lar fish is a sportsman's dream. The stout, fusiform (tapering toward either world record for rod-and-reel bluefin end) body, is nearly round in cross tuna has been established several times section, with a thin tail which is rigid, in Canadian waters. broad and crescent-shaped. Besides its The bluefin tuna is the largest mem- body form, other streamlining features ber of the family Scombridae. In order of the bluefin are its tightly closed jaws; to maintain a high level of activity, these smooth, flat gill covers; eyes that are fish are voracious eaters, feeding mainly "faired in"; and slots into which the fins on other fish. The scombrid family can be retracted. The second dorsal fin supports commercial and recreational and the anal fin are each followed by fisheries averaging 5 million tonnes per a row of 8 to 10 yellow, black-edged year. finlets leading to the base of the tail. It The tunas are highly migratory and is thought that these may reduce turbu- therefore reside in the waters of many lence when swimming. The drawings in countries during their lives. Manage- Figure 2 illustrate the differences ment has been difficult and international between the bluefin and three other agreements must be sought for any tunas that occur off Canada's east effective controls. To assist in bringing coast. Figure 1. Largest bluefin tuna in the about such international management, Often referred to as "warm bodied", world. Landed at Auld's Cove, a number of bodies have been organ- bluefin are able to maintain their body N.S. on October 26, 1979, by ized. For the Atlantic, this body is the temperatures up to 10°C above that of Ken Fraser. Photo by Pat International Commission for the Con- the surrounding water. This is done McKinnon, Port Hawkesbury, servation of Atlantic Tunas (ICCAT) set through the use of a heat exchange sys- N.S. up in 1969. tem, which permits their muscles to The bluefin tuna is widely distributed, operate more efficiently in a much ranging throughout the Atlantic and greater range of water temperature than Pacific oceans. It is highly esteemed in any of their prey species. This also the lucrative Japanese sashimi (fresh, enables them to move to different raw fish) markets for its high oil content climatic areas in search of food. and bright red flesh. World Distribution Description Bluefin tuna occur in the subtropical Because of their wide distribution, and temperate waters of the northern bluefin tuna have many different com- Atlantic and Pacific oceans as well as mon names. In Canadian waters alone, the Mediterranean and the Black seas. they are referred to as horse mackerel, In the Atlantic, they are found from great albacore, tunny, and black fish. Labrador and Newfoundland in the Bluefin are one of the fastest fish in the northwest through the Gulf of Mexico world, capable of bursts of between 70 and the Caribbean to Brazil. In the east, to 88 km per hour. This speed, coupled they are found from Norway to the with their large size, has made them one Mediterranean. Some catches have been of the most esteemed gamefish in the reported off South Africa although world. Canada was the home of the these may have been the southern blue- international Tuna Cup Match, held off fin tuna which is a different species, western Nova Scotia, between 1937 and occurring in the southern oceans. 1976. Now, North Lake, P.E.I. is The migrations of bluefin can be known as the Tuna Capital of the among the most extensive of any fish. World. The largest bluefin caught to Their movements appear associated with date (1985) was landed in 1979 in Auld's water temperature, spawning habits, Cove, N.S. in the Gulf of St. Lawrence. and the endless pursuit of prey. Larger bluefin tend to travel further than smaller individuals. Thus, at the Underwater World 3 extremes of their range, larger fish tend Thunnus thynnus Bluefin tuna (Eng) Thon rouge (Fr) to dominate in numbers. They are nor- mally gregarious, travelling in schools ranging from a few large bluefin to SIZE: many thousands of smaller fish. For the Maximum: over 300 cm first few years of life these fish school according to age and size. Common: 40 to 200 cm Tagging studies and trace-element analysis of vertebrae have indicated a limited exchange of fish between the east and west Atlantic. Returns from tagging studies indicate some individual fish have travelled up to 8,000 km in 50 days. Canadian Distribution Thunnus alalunga Albacore (Eng) Germon (Fr) Adult bluefin return to Canadian waters after spawning in the Gulf of Mexico. Although at present only "giants" (greater than 140 kg) occur SIZE: in Canadian waters, prior to the early Maximum: 120 cm 1960s there were two size-classes. The Common: 40 to 110 cm smaller class, referred to as "jumpers" (20 to 70 kg) arrived later (late August) and left earlier than the giants which tended to arrive in June and leave in October or November, depending on water temperatures. The disappearance of jumpers is thought to be due partly to heavy fishing of the smaller sizes in Thunnus albacares Yellowfin tuna (Eng) Albacore (Fr) the early 1960s and partly due to environmental factors. Migration patterns for bluefin in SIZE: Canadian waters have apparently Maximum: 190 cm changed over time. The controlling Common: 40 to 170 cm influences of such changes are not fully understood although environmental conditions and availability of prey are known to be major factors. There may also be a degree of homing in various stocks. The appearance of bluefin in any area is thought to depend to a large extent on the local movement of prey species such as mackerel and herring. The earliest bluefin arrive off Nova Thunnus obesus Bigeye tuna (Eng) Thon obese (Fr) Scotia's coast in June and July, and by late July they are seen as far north in the SIZE: Gulf of St. Lawrence as Chaleur Bay. Maximum: 190 cm Fishing continues until late October Common: 40 to 170 cm when the stock begins to move out of the Gulf. The last stop is often St. Georges Bay, N.S. before the final departure, presumably back to tropical waters for spawning. Figure 2. A comparison of bluefin and three other tunas that occur off Canada's east coast (reproduced from FAO Species Catalogue, Volume 2: Scombrids of the World). Underwater World 4 World Fishery Tunas and mackerels support both commercial and recreational fisheries around the world. Prime fishing areas include the southwestern and eastern tropical Pacific, and the eastern tropi- cal Atlantic. Much of the commercial fishery is taken by "distant water" fisheries (i.e. boats that range widely from their home ports). Small tunas and mackerel are landed by purse seiners (Fig. 3), drift netters, pole and line boats, longliners and trap nets. The larger members of the tuna family are generally caught by long- lining, trolling, and trap nets. The total worldwide tuna catch aver- ages about 5 million tonnes a year with Figure 3. Purse seiner off the U.S. coast in 1981 fishing for small tuna a recent peak of 6 million tonnes in species for the North American market. Photo by Tom Hurlbut, 1978. The world catch for bluefin tuna DFO, Moncton. is considerably less, ranging between 35,000 and 45,000 tonnes. The Atlantic bluefin portion constitutes approxi- mately half of this catch; the Canadian portion is very small, not exceeding 700 tonnes a year in the last decade (Table 1). The U.S. and Japan have the largest Table 1 tuna catches and markets in the world. West Atlantic Landings Japan is the largest in terms of the blue- fin, with the greatest part of the world's YEAR Canada Canada Canada U.S.A. Japan TOTAL catch (including Canada's) sold on its gear: TRAP HAND PURSE domestic fresh and frozen fish market. 1970 130 151 1200 3800 100 5700 Canadian Fishery 1971 59 88 900 4100 1400 6900 Canadian fishing for tuna began at 1972 29 188 300 3100 300 4100 the turn of the century off Nova Scotia. 1973 144 239 600 1600 1100 4100 This sport fishery shifted as yearly 1974 256 407 100 1600 900 3500 migration patterns changed and as 1975 144 203 300 2900 1500 5200 widespread interests in sport fishing 1976 172 341 300 1900 2900 6000 increased. The first tuna fishing in 1977 368 301 300 2100 3700 6800 Newfoundland waters began in 1957 in 1978 221 208 200 1900 3100 5900 Conception Bay and spread in later 1979 31 214 0 2300 3600 6500 years to Notre Dame Bay. Tuna fishing 1980 47 259 0 1600 3900 5900 in the Gulf of St. Lawrence (in recent 1981 41 279 100 1400 3800 5900 years, the mainstay of the fishery in 1982 68 223 0 600 300 1400 Canada) was first developed off Prince 1983 7 426 0 1300 700 2500 Edward Island in 1967, and was devel- 1984 3 261 0 1200 700 2200 oped in Chaleur Bay in 1973 and 1985 10* 130* 0 — 2660 (TAC) St.
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